1. Field of the Invention
This invention relates generally to valve assemblies for deployment in an internal combustion engine. More specifically, the invention relates to a valve assembly which is starved of oil and which is self-lubricating in order to reduce catalyst poisoning by engine oil constituents.
2. Related Art Statement
Emission reductions in automobile exhaust systems will continue to be required in most passenger and commercial vehicles for the foreseeable future. Contaminants from engine lubricating oil are recognized as a major contributory factor to some types of catalyst poisoning in a catalytic converter.
Engine lubricating oil may pass into the catalytic converter through the engine valve assembly. The conventional valve assembly includes a valve guide which is housed within a cylinder head of an internal combustion engine. Received within the valve guide is a valve having a valve stem and a valve head connected thereto. The valve stem is reciprocatingly engaged within the valve guide. Conventionally, a valve stem seal is disposed around a portion of the valve stem which protrudes beyond the valve guide.
In most engines used today, oil tends to reach the exhaust stream from flow between certain components of the valve assembly--such as the valve guide and the valve stem, or between the valve head and a valve seat. In practice, some flow is generally considered to be necessary so that a relatively friction-free interface is provided between the valve stem and guide during reciprocal movement--especially at high temperatures.
Simply stated, if oil enters the exhaust system, it will burn. However, if some oil does not penetrate into the valve and valve guide interface, the valve guide will wear. By controlling leakage through a valve stem seal, a compromise can be reached between lubricity and catalyst poisoning.
It is known that a valve seat defined within the cylinder head, which cooperates with the valve head, is exposed to an environment having a significantly higher temperature than that to which the valve stem is exposed. To accommodate differential heating, substantial clearance between conventional materials is often needed to avoid seizure. But as the clearance is increased, engine oil may penetrate into the exhaust system between the valve seat and the valve head.
As a practical matter, if seizures can be avoided, smaller dimensional tolerances in today's valve assemblies can be achieved by using stiffer materials having superior thermal conductivity. Such a design approach offers the attribute of better alignment of the valve stem within the valve guide and offers lower maximum bending stresses. When tolerances are reduced, the amount of oil transferred into the exhaust system tends to be diminished.
To satisfy such design criteria, a need has arisen to provide materials which exhibit a low coefficient of thermal expansion and a high thermal conductivity, together with good wear resistance and stiffness. If the valve guide and associated valve materials are selected so that there is a closer match of thermal expansion characteristics, smaller clearances can be designed into the valve assembly, despite the need for member components to operate alternately in hot and cold conditions.
However, in most materials available today, it is relatively unusual to find an acceptable combination of high stiffness and high thermal conductivity. If this relatively unusual combination of characteristics is found, such materials would allow tighter tolerances, less oil loss, better alignment, and lower maximum stresses.
Such opportunities may be afforded by dry-lubricated materials, which are the subject of this invention. When engineered into components such as valve guides, such material will permit close tolerances, so that a dry-lubricated valve guide will result in a reduced level of catalyst poisoning by engine oil constituents.
Of interest is commonly owned U.S. Pat. No. 4,872,432 which issued on Oct. 10, 1989 to Rao, et al. Dr. Rao is a co-inventor of the present application. That reference discloses a gas phase lubrication system which operates effectively within an oilless engine. The gas phase lubrication system includes an annular body of graphite carrying a high temperature solid lubricant within the piston or the cylinder. Also described is an elastomer which is retentive of elasticity at the maximum operating temperature to be experienced, the elastomer being interposed to close the piston-cylinder cap under substantially all operating conditions of the engine. Grooves entrap combustion gases, which function as a bearing over which the piston rides during reciprocation. The disclosure of U.S. Pat. No. 4,872,432 is herein incorporated by reference. In that disclosure, a cylinder wall surface is thoroughly cleansed to remove any oxidation before grit blasting to increase porosity and thereby the reception of a coating. Unlike that disclosure, the approach of the present invention calls for porosity (and adhesion) to be achieved by a different mechanism.
U.S. Pat. No. 4,851,375 issued on Jul. 25, 1989 to Newkirk, et al, and is assigned to Lanxide Technology Company, LP. That reference discloses a method for producing a self supporting ceramic composite structure with a ceramic matrix. The matrix is prepared by oxidation of a parent metal to form a polycrystalline material which consists essentially of the oxidation reaction product. The disclosure of U.S. Pat. No. 4,851,375 is also incorporated here by reference. However, there is no suggestion in that reference of a provision which would enable the metallic phase to be removed and a dry lubricant inserted in its place so that a zero clearance between the mating components can be attained and maintained.
Also of interest is Japanese Patent No. 151708 which was published on Dec. 6, 1980, and is assigned to Nissan Motor KK. That reference discloses a valve guide for an internal combustion engine. The valve guide is formed from a ceramic material consisting of silicon carbide or silicon nitride. That reference suggests that the valve guide exhibits small abrasion losses and requires almost no lubricating oil because of its ceramic properties.
Accordingly, it is an object of the present invention to provide an oil-starved valve assembly for an internal combustion engine, the valve assembly having reduced friction, yet offering considerably reduced leakage of engine oil into the exhaust system.
It is also an object of this invention to control dimensional clearances between the valve and the valve guide or between the valve head and valve seat insert of an internal combustion engine. Such clearances may vary due to thermal variations of the materials used therein and mechanical variations which are attributable to loads imposed on the member components of the valve assembly.
It is a further object of the invention to provide a valve assembly including a valve stem guide which has an internal bore with a solid film lubricant impregnated therewithin so that friction therebetween is reduced and the passage of oil to the exhaust system of the engine is minimized.
It is a still further object of the present invention to provide a solid film lubricant impregnated within a valve seat insert disposed within the engine, thereby further limiting the passage of oil to the exhaust system thereof.
Additionally, it is another object of the invention to provide a solid film lubricant which comprises a composite of a solid lubricant impregnated into an oxide-metal material.
Furthermore, it is an object of the invention to provide a composition of matter including an oxide-metal material and a dry lubricant, wherein the oxide-metal material forms a matrix having anchoring sites for retaining the dry lubricant therewithin, the resulting composition having the characteristic of lubricity at elevated temperatures.
Also, it is an object of the present invention to provide a process for preparing a composition of matter, preferably a valve assembly, the assembly including a valve stem guide with an intermixed phase composite of a metal, a metal oxide, and silicon carbide, the process calling for a fluid lubricant in the form of a resin suspension of boron nitride, molybdenum disulfide, graphite, and mixtures thereof.